The present invention is generally directed to fuel cells and more specifically to fuel cell systems and their operation.
Fuel cells are electrochemical devices which can convert energy stored in fuels to electrical energy with high efficiencies. High temperature fuel cells include solid oxide and molten carbonate fuel cells. These fuel cells may operate using hydrogen and/or hydrocarbon fuels. There are classes of fuel cells, such as the solid oxide regenerative fuel cells, that also allow reversed operation, such that oxidized fuel can be reduced back to unoxidized fuel using electrical energy as an input.
In a high temperature fuel cell system such as a solid oxide fuel cell (SOFC) system, an oxidizing flow is passed through the cathode side of the fuel cell while a fuel flow is passed through the anode side of the fuel cell. The oxidizing flow is typically air, while the fuel flow is typically a hydrogen-rich gas created by reforming a hydrocarbon fuel source. The fuel cell, operating at a typical temperature between 750° C. and 950° C., enables the transport of negatively charged oxygen ions from the cathode flow stream to the anode flow stream, where oxygen from oxygen ions combines with either free hydrogen or hydrogen in a hydrocarbon molecule to form water vapor and/or with carbon monoxide to form carbon dioxide. The excess electrons from the negatively charged ion are routed back to the cathode side of the fuel cell through an electrical circuit completed between anode and cathode, resulting in an electrical current flow through the circuit.
In most applications of high temperature fuel cells, a number of cells are connected in series forming a cell “stack.” Cells connected in series operate at the same current and are thereby locked to the same operating state. In order to operate efficiently, these fuel cells in the stack have to be maintained at or near their nominal or designed operating temperature. The fuel cells are usually designed to operate at a high or even a maximum load at which the fuel cells achieve the nominal or designed operating temperature. Thus, under high electrical loads, high temperature fuel cell stacks dissipate enough heat to maintain their nominal or designed operating temperature (provided adequate thermal management/insulation). However, sometimes the fuel cells operate at a partial or low load, which is lower than the designed high or maximum operating load. This may occur when there is low power demand on the fuel cell stack, for example. At partial or low load, the amount of heat dissipated quickly tapers off and the high temperature fuel cell stack may drop in operating temperature. Generally, a drop in operating temperature may be acceptable, but at lower temperature the stack has reduced power generation capabilities and may be unable to provide sufficient power when the power demand increases suddenly.